<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container {
            width: 1226px;
            margin: auto;
        }

        .a {
            width: 100px;
            height: 100px;
            background: red;
        }

        .a1 {
            width: 100%;
            height: 50px;
            background-color: #ff0;
            position: absolute;
            top: 100px;
            left: 0;

            height: 0;
            overflow: hidden;
            transition: 1s;

            

        }


        .a:hover .a1 {
            height: 50px;
        }
    </style>
</head>

<body>




    <div class="a">
        <div class="a1">
            <div class="container">
                <!-- <ul>li</ul> -->
            </div>
        </div>
    </div>
</body>

</html>